How plaster of Paris is made

Plaster of Paris is gypsum cooked just hot enough to lose most of its water, so adding water brings it back to stone.

stations: 5
things upstream: 1
raw source: 1
step from the ground: 1

The short version

Plaster of Paris is made by heating gypsum to about 120 to 180 °C in a kiln. The mineral loses most of its bound water as steam and becomes a white powder. Mixed with water, the powder starts to set in about 10 minutes and is hard in about 45, as it returns to gypsum.

Quarry the gypsum

Gypsum is a soft sulfate mineral found in thick evaporite beds. It is widely mined from open pits and underground workings.

Goes in
Gypsum
Crush the rock

Quarried lumps are broken into small pieces so that heat can reach all the way through them.
Heat it gently in a kiln

Heated to about 120–180 °C, gypsum loses part of its water as steam and becomes calcium sulfate hemihydrate. Overheat it past 250 °C and it turns into dead-burned plaster, which no longer sets quickly.

120–180 °C
Grind to a fine powder

The cooked gypsum is ground into a white powder that dissolves and recrystallizes quickly when it meets water.
Seal it from moisture

Plaster is stored in moisture-proof containers, since even humid air starts to hydrate it slowly and ruins it.

End of the linePlaster of Paris Next it goes into gold ring, prosthetic leg.

Worth repeating

The name comes from a large gypsum deposit at Montmartre, a hill in the north end of Paris.
In ancient Egypt, burnt and crushed gypsum was mixed with water to make mortar between the blocks of pyramids.
Plaster expands slightly as it sets, which helps it fill every detail of a mold.